[clang-tools-extra] [Clang-Tidy][NFC] Simplify check cppcoreguidelines-missing-std-forward (PR #138504)

[Yanzuo Liu via cfe-commits]

https://github.com/zwuis commented:

> Strange, can you explain why that's the case?

`CaptureInCopy` matches lambda expressions whose default capture kind is copy and which have an explicit capture referencing function parameter.

`CaptureByRefExplicit` matches lambda expressions which have an explicit capture captured by reference and referencing function parameter, not requiring default capture kind.

> Are we missing tests?

I don't think we miss tests.

https://github.com/llvm/llvm-project/pull/138504